looking at them the are a balanced flue design like your boiler at home, maybe difficult to locate in a bus because balanced flue's have a max length and restriction on bends etc,
http://www.wallas.fi/contentlibrary/PDF/asennus_18002400.pdf" onclick="window.open(this.href);return false;
starts in finish but has english about page 16/17. they don't look ideal for buses